1. Field of the Invention
The invention refers to a heart stimulation system for stimulating at least one chamber of a heart by means of electrical stimulation pulses that are delivered when a delay time started by a cardiac event expires. The invention particularly refers to implantable medical devices such as implantable pacemakers and implantable cardioverter/defibrillators for atrial synchronous stimulation of a ventricle of a heart.
2. Description of the Related Art
Implantable heart stimulators can be used for cardiac rhythm management (CRM) for treating a variety of heart functional and rhythm disorders including but not limited to bradycardia, tachycardia or fibrillation by way of electric stimulation pulses delivered to the heart tissue, the myocardium. A sufficiently strong stimulation pulse outside a heart chamber's refractory period leads to excitation of the myocardium of that heart chamber, which in turn is followed by a contraction of the respective heart chamber.
Depending on the disorder to be treated, such heart stimulator generates electrical stimulation pulses that are delivered to the heart tissue (myocardium) of a respective heart chamber according to an adequate timing regime. Delivery of stimulation pulses to the myocardium is usually achieved by means of an electrode lead that is electrically connected to a stimulation pulse generator inside a heart stimulator's housing and that carries a stimulation electrode in the region of its distal end. A stimulation pulse also is called a pace. Similarly, pacing a heart chamber means stimulating a heart chamber by delivery of a stimulation pulse.
In order to be able to sense the contraction a heart chamber which occurs naturally without artificial stimulation and which is called an intrinsic contraction, the heart stimulator usually includes at least one sensing stage that is connected to a sensing electrode and said electrode is placed in or near the heart chamber. An intrinsic excitation of a heart chamber results in characteristic electrical potentials that can be picked up via the sensing electrode and that can be evaluated by the sensing stage in order to determine whether an intrinsic excitation—called: intrinsic event—has occurred.
Usually, a heart stimulator features separate stimulation pulse generators for each heart chamber to be stimulated. Therefore, in a dual chamber pacemaker, usually an atrial and a ventricular stimulation pulse generator for generating atrial and ventricular stimulation pulses are provided. Delivery of an atrial or a ventricular stimulation pulse causing an artificial excitation of the atrium or the ventricle, respectively, is called an atrial stimulation event AP (atrial paced event) or a ventricular stimulation event VP (ventricular paced event), respectively.
Similarly, common heart stimulators feature separate sensing stages for each heart chamber to be of interest. In a dual chamber pacemaker usually two separate sensing stages, an atrial sensing stage and a ventricular sensing stage, are provided that are capable to detect intrinsic atrial events AS (atrial sensed event) or intrinsic ventricular events VS (ventricular sensed event), respectively.
As known in the art, separate sensing and pacing stages are provided for three-chamber (right atrium RA, right ventricle RV, left ventricle LV) or four-chamber (right atrium RA, left atrium LA, right ventricle RV, left ventricle LV) pacemakers or ICDs.
By means of a sensing stage for a heart chamber to be stimulated, the pacemaker is able to only trigger stimulation pulses when needed that is when no intrinsic excitation of the heart chamber occurs in the allotted time. Such mode of pacing a heart chamber is called demand mode. In the demand mode the pacemaker schedules an atrial or a ventricular escape interval that causes triggering of an atrial or ventricular stimulation pulse when the escape interval times out. Otherwise, if an intrinsic atrial or ventricular event is detected prior to time out of the respective atrial or ventricular escape interval, triggering of the atrial or ventricular stimulation pulse is inhibited. Such intrinsic (natural, non-stimulated) excitations are manifested by the occurrence of recognizable electrical signals that accompany the excitation and depolarization of a cardiac muscle tissue (myocardium). The depolarization of the myocardium is usually immediately followed by a cardiac contraction. For the purpose of the present application, depolarization and contraction may be considered tightly coupled events and the terms “depolarization” and “contraction” are used herein as synonyms.
In a heart cycle, an excitation of the myocardium leads to a depolarization of the myocardium that leads to a contraction of the heart chamber. If the myocardium is fully depolarized it is unsusceptible for further excitation and is thus refractory. Thereafter, the myocardium repolarizes and thus relaxes and the heart chamber expands again. In a typical intracardiac electrogram (iEGM) depolarization of the ventricle corresponds to a signal known as the “R-wave”. The repolarization of the ventricular myocardium coincides with a signal known as the “T-wave”. Atrial depolarization is manifested by a signal known as the “P-wave”.
A natural contraction of a heart chamber can be similarly detected by the evaluating electrical signals sensed by the sensing channels. In the sensed electrical signal the depolarization of an atrium muscle tissue is manifested by occurrence of a P-wave. Similarly, the depolarization of ventricular muscle tissue is manifested by the occurrence of a R-wave. The detection of a P-wave or a R-wave signifies the occurrence of intrinsic atrial, As, or ventricular, Vs events, respectively.
Several modes of operation are available in state of the art multimode cardiac stimulation systems. The modes of operation are concerned with which chambers of the heart are monitored for native activity; which chambers of the heart are provided with pacing therapy; and which cardiac functions are treated: inter-chamber conduction (AV Node, nodal disease); chronotropy (sinus node, sinus disease); and intra-chamber conduction (bundle branch delays and blocks, conduction disorders). The operational modes of a stimulation system, single, dual or multi-chamber devices, are classified using a standard descriptive code. This invention covers the automatic characterization and determination of the timing objectives for managing inter-chamber and intra-chamber disorders.
The NBG code system is used to describe these modes of operations. In such a code, the first three letters describe the primary configuration. The First letter identifies the chamber of the heart that is paced (i.e., that chamber where a stimulation pulse is delivered), with a “V” indicating the ventricle, an “A” indicating the atrium, and a “D” indicating both the atrium and ventricle. The second letter of the code identifies the chamber wherein cardiac activity is sensed, using the same letters, and wherein an “O” indicates no sensing occurs. The third letter of the code identifies the action or response that is taken by the pacemaker. In general, three types of action or responses are recognized: (1) an Inhibiting (“I”) response wherein a stimulation pulse is delivered to the designated chamber at the conclusion of the appropriate escape interval unless cardiac activity is sensed during the escape interval, in which case the stimulation pulse is inhibited; (2) a Trigger (“T”) response wherein a stimulation pulse to a prescribed chamber of the heart following a prescribed period of time after a sensed event; or (3) a Dual (“D”) response wherein both the Inhibiting mode and Trigger mode may be evoked, e.g., with the “inhibiting” occurring in one chamber of the heart and the “triggering” in the other.
To the primary three-letter code, a fourth letter “R” may optionally be added to designate a rate-responsive pacemaker and/or whether the rate-responsive features of such a rate-responsive pacemaker are enabled (“O” typically being used to designate that rate-responsive operation has been disabled). A rate-responsive pacemaker is one wherein a specified parameter or combination of parameters, such as physical activity, the amount of oxygen in the blood, the temperature of the blood, etc., is sensed with an appropriate sensor and is used as a physiological indicator of what the pacing rate should be. When enabled, such rate-responsive pacemaker thus provides stimulation pulses that best meet the physiological demands of the patient.
To the fore mentioned first 4-letter code, a fifth code field may be added to designate the inter-chamber configuration.
A dual chamber pacemaker featuring an atrial and a ventricular sensing stage and an atrial and a ventricular stimulation pulse generator can be operated in a number of stimulation modes like VVI, wherein atrial sense events are ignored and no atrial stimulation pulses are generated, but only ventricular stimulation pulses are delivered in a demand mode, AAI, wherein ventricular sense events are ignored and no ventricular stimulation pulses are generated, but only atrial stimulation pulses are delivered in a demand mode, or DDD, wherein both, atrial and ventricular stimulation pulses are delivered in a demand mode. In such DDD mode of pacing, ventricular stimulation pulses can be generated in synchrony with sensed intrinsic atrial events and thus in synchrony with an intrinsic atrial rate, wherein a ventricular stimulation pulse is scheduled to follow an intrinsic atrial contraction after an appropriate atrioventricular delay (AV-delay; AVD), thereby maintaining the hemodynamic benefit of atrioventricular synchrony.
The AV-delay determines the chronological relation between an atrial event and a prescribed point of time of a ventricular event, the ventricular escape interval.
Since an optimal AV-delay may vary for different heart rates or stimulation rates and may even vary from patient to patient, the AV-delay usually is adjustable.
In order to promote natural ventricular events, often the ventricular escape interval is extended by a short time interval thus resulting in a prolonged ventricular escape interval called “AV hysteresis interval”